In the past, fluid from cooling towers was directly discharged without the benefit of treatment or recycling for reuse the cooling tower. However, since the 1970's, such discharge has been subject to EPA pre-treatment standards and NPDES effluent limitations, as well as local regulations. Additionally, such “once through” water usage leads to high water and sewage cost.
In response to regulatory restrictions and water cost, water filtration devices have been developed to treat cooling tower discharge for recycling back to the cooling tower. It is important that the water quality from such treatment devices be sufficient to prevent scaling, erosion, and biological fouling in the cooling tower. Of particular importance is the removal efficiency of suspended solids. The concentration of suspended solids in cooling tower inlet water effects the number of times that in which the water can be recycled before solids precipitate from solution. Clearly, water with high initial suspended solids can be recycled only a minimal number of times before precipitation occurs in comparison to more purified water.
Two common treatment devices which provide full flow filtration of process water from a cooling tower are centrifugal separators and sand bed filters. Centrifugal separators tangentially introduces flow water into a cylindrical housing, causing the water to swirl. Suspended solids with a specific gravity greater than that of water migrate to the perimeter wall of the cylinder where they are assisted downward by gravity into a collection sump and is periodically removed as sludge. This process is limited in that it has a relatively poor removal efficiency for suspended solids having a specific gravity moderately above that of water, and for smaller solids such as those less than 45 microns in diameter.
Sand bed filters operate by passing a fluid through filtering sand contained within a vessel. Suspended solids within the fluid are captured by the sand while the cleansed fluid passes there-through. Sand bed filters are particularly effective in removing suspended solids, including those of a relatively small size. When the sand begins to clog, a pressure differential switch signals a backwash cycle that reverses the flow through the sand thereby lifting it and carrying captured particulate matter away from the sand filter for proper disposal.
Current sand filter technology introduces the backwash flow at a distance above the bottom of the treatment vessel by employing a header and lateral system. The header and lateral configuration inherently has gaps between the laterals and, consequently, has spaces where no flow occurs making it difficult, if not impossible, to fully fluidize the filter bed. Additionally, the space below the header and laterals to the bottom of the vessel are provided with no, or only minimal, flow. Accordingly, present filter beds become clogged with particulate matter which decreases the useful life of the treatment device and cooling tower. Further, as the filter bed becomes clogged, the pressure head across the filter increases, reducing the capacity of the filter to treat fluids in terms of fluid through rate and suspended solids removal efficiency. The poisoned bed, impregnated by particulate matter, must be replaced before continued use.
What is needed is a water treatment device that is highly efficient in removing suspended solids. Also needed is for a water treatment device that is fully fluidized during backwashing, thereby providing a long effective life. Also needed is for the water treatment device to be small in size yet capable of treating process water at a high flow rate.